LIPIDS

 

LIPIDS

PREPARED BY MR. ABHIJIT DAS

Lipids are organic compounds formed from fatty acids and alcohol, and they are hydrophobic in nature.

CLASSIFICATION

Lipids are broadly classified into three main categories based on their structure and composition: simple lipids, conjugated lipids, and derived lipids.

1.     SIMPLE LIPIDS: These are esters formed from the combination of fatty acids with various alcohols. The primary types of simple lipids include triglycerides (fats and oils), which are made up of three fatty acid molecules linked to a glycerol molecule. Waxes are another type of simple lipid, usually consisting of long-chain fatty acids bonded to long-chain alcohols (ex: Cetyl alcohol)

2.     CONJUGATED LIPIDS: These lipids are characterized by the addition of further groups to the basic molecule of simple lipids. Complex lipids include phospholipids, glycolipids, and lipoproteins. Phospholipids have a phosphate group in their structure. Glycolipids are lipids with a carbohydrate attached. Lipoproteins are compounds containing both lipid and protein components.

3.     DERIVED LIPIDS: These are compounds derived from simple and complex lipids through various chemical processes. Examples of derived lipids include fatty acids, sterols (such as cholesterol).

TRIGLYCERIDES

STRUCTURE

Triglycerides are a type of lipid, classified as a simple lipid. They are composed of three fatty acid molecules bonded to a glycerol molecule. This chemical structure is what gives triglycerides their name ("tri-" meaning three, and "glyceride" referring to the glycerol backbone).

PROPERTIES

1.     Energy Storage: Triglycerides store extra calories as a source of energy for the body.

2.     Hydrophobic: They don't mix with water and are hydrophobic.

3.     Insulation: Triglycerides under the skin act as insulation to maintain body temperature.

4.     Storage of Vitamins: They help store and absorb fat-soluble vitamins (A, D, E, K).

5.     Metabolism: Triglycerides can be broken down to release stored energy when needed.

FATTY ACID

A fatty acid is a simple molecule made up of a hydrocarbon chain with a carboxylic acid group at one end.

Ex: Palmitic acid

CLASSIFICATION (BASED ON CHEMICAL CHARACTERISTICS)

a.     Saturated Fatty Acids: These fatty acids contain only single bonds between carbon atoms and have no double bonds in their hydrocarbon chain. Examples include stearic acid.

b.     Unsaturated Fatty Acids: These fatty acids have at least one double bond in their hydrocarbon chain.

Ø Monounsaturated Fatty Acids: Have one double bond, like oleic acid.

Ø Polyunsaturated Fatty Acids: Contain more than one double bond, such as linoleic acid or alpha-linolenic acid.

CLASSIFICATION (BASED ON NUTRITIONAL REQUIRMENTS)

Fatty acids can be classified based on nutritional requirements into two primary categories:

1.     Essential Fatty Acids (EFAs): These are fatty acids that the human body cannot synthesize on its own, so they must be obtained from the diet. The two main types of essential fatty acids are:

a. Omega-3 Fatty Acids: Such as alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Sources include fatty fish (salmon, mackerel), flaxseeds, chia seeds, and walnuts.

b. Omega-6 Fatty Acids: Like linoleic acid. Sources include certain vegetable oils like sunflower, safflower, and corn oil.

2.     Non-Essential Fatty Acids: These are fatty acids that the body can synthesize on its own, so they are not required in the diet. These include saturated fatty acids and some monounsaturated fatty acids.

CHOLESTEROL

STRUCTURE

The chemical formula for cholesterol is C₂₇H₄₆O. It consists of four interconnected hydrocarbon rings with a hydroxyl (OH) group attached to one end.

FUNCTIONS

1.     Cell Membrane stability: Cholesterol helps maintain the structural integrity and fluidity

of cell membranes in the body's cells.

2.     Hormone Production: It serves as a precursor for the synthesis of various hormones, including cortisol, estrogen, and testosterone.

3.     Vitamin D Synthesis: Cholesterol is a precursor to vitamin D synthesis when exposed to sunlight.

4.     Bile Production: Cholesterol is a component of bile acids that aid in the digestion and absorption of fats in the intestines.

LIPOPROTEINS

Lipoproteins are composed of a mixure of proteins and fats. They’re carriers in the blood that transport fats, including cholesterol and triglycerides, around the body, as fats alone can't move freely in the bloodstream.

TYPES

1.     Chylomicrons: They transport dietary triglycerides from the intestines to various tissues.

2.     Very-Low-Density Lipoproteins (VLDL): These lipoproteins transport newly synthesized triglycerides from the liver to other tissues. They are composed mainly of triglycerides.

3.     Low-Density Lipoproteins (LDL): LDL particles carry cholesterol from the liver to tissues throughout the body. Often termed as "bad cholesterol," high levels of LDL are associated with an increased risk of cardiovascular diseases.

4.     High-Density Lipoproteins (HDL): HDL particles transport cholesterol from the tissues back to the liver, where it can be broken down and removed from the body. Often referred to as "good cholesterol," higher levels of HDL are linked to a reduced risk of heart disease.

FUNCTIONS

1.     Transport of Lipids: Lipoproteins transport fats, cholesterol, and fat-soluble vitamins throughout the body, allowing these essential substances to move in the bloodstream to tissues and cells where they are needed.

2.     Energy Transport: Lipoproteins transport triglycerides, a major energy source, to muscles and other tissues.

3.     Cell Membrane Maintenance: Some lipoproteins contribute to the maintenance and repair of cell membranes by delivering phospholipids and cholesterol.

4.     HDL's Protective Role: High-density lipoproteins (HDL) have a protective effect by removing excess cholesterol from the bloodstream, thus potentially reducing the risk of cardiovascular diseases.

FUNCTIONS OF LIPIDS

1.     Energy Storage: When the body has excess calories, they are converted into triglycerides and stored in fat cells for later energy use.

2.     Cellular Structure: Lipids, especially phospholipids, are essential components of cell membranes. They help form the structure of cells.

3.     Insulation: Certain lipids provide insulation, helping to regulate body temperature and protect organs.

4.     Hormone Production: Lipids are essential in the production of hormones. Steroid hormones are derived from cholesterol.

5.     Vitamin Absorption: Fat-soluble vitamins (A, D, E, and K) require lipids for absorption in the body. Lipids assist in the absorption and transportation of these essential vitamins.

6.     Protection: Some lipids act as a protective layer for organs and nerves. For example, myelin, composed partly of lipids, insulates nerve fibers, allowing efficient nerve impulse transmission.

QUALITATIVE TESTS OF LIPIDS

1.     Grease Spot Test: This test involves placing a drop of the substance on filter paper. If the substance leaves a translucent grease spot after the solvent evaporates, it indicates the presence of lipids.

2.     Sudan Red Test: Sudan Red is a dye that specifically stains lipids. When added to a sample suspected to contain lipids, the appearance of a red color indicates the presence of lipids.

3.     Emulsion Test: This test involves shaking the sample with water. If the solution turns milky or cloudy, it suggests the presence of lipids. The cloudiness is due to the formation of an emulsion of tiny lipid droplets suspended in water.

4.     Iodine Test: Iodine can be used to test for the presence of lipids, particularly unsaturated lipids. When iodine is added to a sample, it will change color (from brown to purple) in the presence of unsaturated lipids, such as fats and oils.

5.     Benedict's Test: This test is used specifically for the identification of reducing sugars but can indirectly indicate the presence of lipids. If a milky or cloudy precipitate forms upon heating the sample with Benedict's solution, it suggests the presence of lipids in the presence of reducing sugars.

METABOLISM OF LIPIDS

LIPOLYSIS:

Ø Lipolysis is the metabolic process through which triglycerides are broken down into fatty acids and glycerol.

Ø Fatty Acid Utilization for ATP Production: Fatty acids undergo beta-oxidation within cells to generate acetyl-CoA, which enters the citric acid cycle (Krebs cycle) and eventually leads to the production of ATP.

Ø Glycerol Conversion to Glucose: Glycerol, the other product of lipolysis, can be converted into glucose through a process known as gluconeogenesis.

BETA OXIDATION OF FATTY ACID (PALMITIC ACID):

1.     Activation: Palmitic acid first undergoes activation in the cytoplasm, where it combines with CoA to form palmitoyl-CoA. This step requires an input of energy in the form of ATP.

2.     Transport into the Mitochondria: Palmitoyl-CoA is transported into the mitochondria, crossing both the outer and inner mitochondrial membranes.

3.     Beta-Oxidation (Repeating Cycle):

·         Step 1: The palmitoyl-CoA undergoes a reaction in which the first two carbons are cleaved off, producing acetyl-CoA and a shortened fatty acyl-CoA chain (now 14 carbons long).

·         Step 2: The shortened fatty acyl-CoA chain undergoes another round of beta-oxidation, releasing another acetyl-CoA and further shortening the chain.

This process repeats until the entire fatty acid is broken down into multiple acetyl-CoA molecules.

4.     Acetyl-CoA Enters the Citric Acid Cycle: The acetyl-CoA produced in each round of beta-oxidation enters the citric acid cycle, where it is further metabolized to produce NADH, FADH2, and GTP, which contribute to the production of ATP in the electron transport chain.

 

KETOGENESIS:

Definition: Ketogenesis is the conversion of fatty acids into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) by liver.

When Ketogenesis Occurs:

1.     Fasting: When the body is in a fasted state, such as during extended periods without food or during overnight sleep, insulin levels drop, and glucagon and other counter-regulatory hormones rise. This hormonal shift signals the body to break down stored fats into fatty acids, leading to ketogenesis.

2.     Low-Carbohydrate Diet: In a low-carbohydrate or ketogenic diet, where carbohydrate intake is restricted, the body's primary source of energy, glucose, becomes limited. As a result, the body shifts to using fats as the primary fuel source, leading to increased ketogenesis.

3.     Exercise: During prolonged and intense exercise, especially when glycogen stores become depleted, the body relies more on fatty acids for energy. This can also trigger ketogenesis to provide an alternative energy source.

KETOLYSIS:

Ketolysis is the metabolic process through which ketone bodies are broken down and utilized for energy.

1.     Ketone Body Production: Ketolysis occurs after ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) are produced during ketogenesis, a process that takes place in the liver. Ketogenesis usually occurs in response to low carbohydrate availability, such as during fasting, a low-carbohydrate diet, or intense exercise.

2.     Transport of Ketone Bodies: After their production, ketone bodies are released into the bloodstream and transported to tissues that can utilize them for energy.

3.     Entry into Cells: Once in the cells, ketone bodies are broken down in a process called ketolysis. This involves the conversion of ketone bodies back into acetyl-CoA, a molecule that enters the citric acid cycle (Krebs cycle) within the mitochondria.

4.     Energy Production: Acetyl-CoA generated from ketolysis is then used for energy production through oxidative phosphorylation, ultimately leading to the synthesis of adenosine triphosphate (ATP).

DISEASES RELATED TO ABNORMAL METABOLISM OF LIPIDS:

1.  KETOACIDOSIS

Ketoacidosis is a serious metabolic condition characterized by high levels of ketone bodies in the blood, leading to increased acidity.

Dehydration in Ketoacidosis: Dehydration is a common feature of ketoacidosis. The breakdown of fats for energy during ketoacidosis produces ketones, which are acidic. The body attempts to eliminate these ketones through increased urination, leading to significant fluid loss and dehydration. This can be life threatening.

2.  FATTY LIVER

Definition: Fatty liver, also known as non-alcoholic fatty liver disease (NAFLD), is a condition characterized by the accumulation of excess fat in the liver cells. This buildup of fat can lead to inflammation and liver damage over time.

Reasons for Fatty Liver:

1.     Obesity:

·         Excess body weight, especially visceral fat around the abdomen, is a significant risk factor for fatty liver.

2.     Type 2 Diabetes:

·         Individuals with type 2 diabetes often have insulin resistance, which can contribute to the accumulation of fat in the liver.

3.     Dietary Factors:

·         Diets high in refined carbohydrates, sugars, and saturated fats can contribute to the accumulation of fat in the liver.

4.     Sedentary Lifestyle:

·         Lack of physical activity is associated with obesity, insulin resistance, and an increased risk of fatty liver.

3.  HYPERCHOLESTEROLEMIA

Hypercholesterolemia is a medical condition characterized by elevated levels of cholesterol in the blood.

REASONS:

1.     Dietary Choices: Consuming a diet high in saturated fats, commonly found in processed foods and fried items, can elevate cholesterol levels.

2.     Lack of Exercise: A sedentary lifestyle contributes to higher cholesterol levels. Regular physical activity helps maintain healthy cholesterol levels.

3.     Genetics: Inherited factors, known as familial hypercholesterolemia, can lead to elevated cholesterol levels even in individuals with a healthy lifestyle.